Method for producing a completely or partially enameled component

ABSTRACT

A method for producing an at least partially enameled component shall enable a particularly high product quality with a particularly low energetic expenditure and a low environmental load. For this purpose, according to the invention, a workpiece ( 2 ) carrying an enamel powder is inductively heated, the operating frequency of the inductor ( 4 ) being chosen, in view of the material properties of the workpiece ( 2 ), such that the electromagnetic depth of penetration into the workpiece ( 2 ) is at most 1 mm.

The invention related to a method for producing an at least partially enameled component.

Enameling is a technologically relatively expensive process, providing, as a result, a highly stressable, chemically neutral, corrosion-resistant, highly insulating and hygienically high-quality composite material. There are many applications in which enameled surfaces offer particular advantages, also in view of a long-time stability.

Enameled components are usually produced by applying enamel slip or enamel powder on a suitably chosen carrier body, after suitable pre-treatments, melting them on and burning them in at temperatures in the range of approximately 800° C. to 900° C. A multitude of methods are available for that purpose. At present, an enameling process consists, as a rule, in cleaning, pre-drying and optional pre-heating of the carrier body and subsequently applying substrates (in particular silicates)—all of that partially in several stages—onto the respective surface, usually a metallic surface, of the carrier body—preferably steel or cast iron—and then heating the surface prepared in this way, by radiating heat onto it, with the substrate or enamel powder until the melting temperature of the enamel powder or enamel slip is reached and the substrate or enamel powder “melted on” enters into an intimate connection with the carrier material at approx. 820° C.

It is true that such an enameling process provides, as a result, a high-quality, versatile product. On the other hand, however, such a production is relatively energy-consuming and resource-intensive. In particular, the melting-on and burning-in processes are energetically particularly expensive.

The invention is, therefore, based on the problem to provide a method for producing an at least partially enameled component which enables a particularly high product quality with a particularly low energetic expenditure and a low environmental load.

This problem is solved according to the invention by inductively heating a carrier body carrying an enamel powder, the operating frequency of the inductor being chosen, in view of the material properties of the workpiece, such that the electromagnetic depth of penetration into the workpiece is at most 1 mm.

The invention starts out on the consideration that a high product quality can be assured by reliably observing the general parameters relevant for the enameling process, in particular the local transformation temperatures, when “melting on” the enamel powder or substrate material onto the carrier body. By specifically observing this marginal condition, the required energy demand—and, therewith, also the accompanying environmental load and consumption of resources—can be kept particularly low, by limiting and reducing, in the manner of a focusing, the energy application during the transformation or the melting process, to a local spatial area, in which the desired transformation process shall actually take place. Deliberately departing from the usual method, in which the carrier body carrying the enamel powder is heated and heated up in a furnace globally and, thus, in its full volume and over a large surface, it is now provided to heat up, in a locally limited, focused manner, a limited spatial area of the carrier body, to which the intended material transformation shall be limited. This can be achieved in a particularly simple way by locally heating the carrier body through inductive heating.

The heat necessary for the enameling process is, therefore, not applied to the substrate surface via a third medium and then, via the surface, to the interface of carrier material and substrate. Instead, the relevant spatial area, in particular at the interface of carrier body and substrate or enamel powder, is inductively heated directly and immediately. This results in the fact that during the formation process of the enamel layer (burning in/curing), the latter is built up directly and growingly from the carrier towards the surface and not—as with the usual method—from the surface towards the carrier body.

The connection between the thermal glaze and the metallic carrier material, produced by the enameling process, is clearly improved in this way, because the energy or heat deposition develops from inside. This results in a clearly improved anchorage of the enamel layer on the carrier body and a new enameling quality. In particular, the adhesion of the substrate or enameling material on the carrier material—even at “critical spots” —is further improved, and a chipping off, in particular at corners or edges, is further clearly reduced.

Furthermore, any occlusions of air or other humidities or other occlusions can escape, during the layer-formation process or the subsequent curing process, from the interface of the carrier body to the outside, because the surface is still “open” during the layer-formation process. Any possible problems caused by undesired bubble-like occlusions of air or gas are minimized.

By applying a second and/or third substrate or material layer, specific contours can be applied and melted with the lower substrate layer. This can specifically be utilized, e.g., for advertisements. For logos on the lower layer, for example, a substrate of a different color/type (e.g. screen printing/templates) can be sprayed on and immediately melted on and cured by tracking a correspondingly designed inductor (if necessary, a further inductor). Through skillful variation and combination, no boundaries are set to creativity and diversity.

Prior to the burning-in process properly speaking, the inductive heating can also be utilized, with another inductor or else with the same inductor (with less power input or shorter exposure time), for pre-drying or pre-heating for special substrate applications, whereby in particular an improved adhesion can be achieved. An inductive drying between the individual cleaning states is also possible.

The inductive heating considerably reduces the necessary process times and powers and, as a consequence of the corresponding process improvements, also reduces the expenditure for equipment and process technology, also in these stages which are actually preceding the burning in.

The inductive heating is used in a particularly preferred manner for repair purposes, whereby in particular individual defective spots in an enamel coating are, for example, locally provided with an enameling paste and then the component needing repair is inductively heated locally, thus applying the “replacement enamel”.

The surfaces and spaces properly speaking which might have to be provided for any still necessary furnaces are clearly reduced—provided that the “chambers” are well insulated. Thanks to the induction, a very local and purposeful heating is possible. A heating over a wide area, which has so far been absolutely necessary, with many energetic losses, can thus be avoided. That is energetically favorable and environmentally friendly.

As compared with the method making use of laser technology for the melting-on and burning-in processes and the striped structure occurring due to this principle, the inductive solution—for example when scanning large parts—does not show this disadvantage. Rather does it allow soft transitions and a line-in-line melting of the coating, which finally results in a surface “of one piece”.

Through the inductive heating of the carrier body, this heating is effected so as to be locally limited to a spatial area of the carrier body, so that through a corresponding selection of the positioning of the energy input, a selective, object-specific process guidance is possible. In addition to other variants, a sequential sweeping of the surface regions of the carrier body in the manner of a “scanning” is also possible. Depending on size and shape of the part, it is possible to scan and heat the body either completely over a wide area or in sections. In this case, the thermal conduction in the metal is in particular effective, which, however, has only little effect on a localized “burning-in center” and is rather an advantage than a disturbance, thus corresponding rather to a kind of pre-heating or basic heating.

In general, hardly any boundaries are set to the design and implementation of the inductor. In case of an enameling all around of disk material or receptacles (e.g. enameled container/storage), ring inductors are absolutely imaginable or advantageous. The surface to be treated of very large area elements can be scanned and thus heated by means of an “area inductor” by means of HF.

For smaller advertising media or for jewelry, a toroid coil or a formed toroid inductor (rectangular/oval or the like) might be advantageous. Optimations in this respect can be determined through tests.

Size and nature of the parts to be enameled also decisively determine the selection of the frequency for the inductive heating (MF or HF); in many applications, it will certainly be favorable to select an HF, because in this case, the interface to the substrate is specifically addressed/heated (skin effect).

The local limitation of the induction effect, which is optionally also possible, in particular through a suitable process guidance, enables, furthermore, to a particular high degree an efficient and cost-advantageous solution in the repair sector when reprocessing surfaces which are completely or partially scratched, chipped or damaged in another way, in particular of industrial parts or high-quality consumer goods. Such a repair can also be carried out particularly advantageously in the manner of an in-situ treatment directly at the place of installation, i.e. without requiring to disassemble and re-assemble the respective component. The defect needing treatment is preferably cleaned mechanically and/or chemically prior to the treatment properly speaking and freed from corrosion or other pollutions. After a suitable pre-treatment, for example in the manner of a priming or the like, enamel filler, whose shape and/or color are preferably matched to the component needing treatment, is applied and then inductively and quickly melted on and immediately burned in, in and with the existing “original environment”, i.e. the locally still existing coating residues. The contours melt and merge with each other, so that a homogeneous overall impression of the repaired surface is given. The repaired partial area can be matched to the existing original surface regarding color and/or surface properties, through a suitable selection of the material, in particular the enamel filler, or else be optically emphasized purposefully, so that it can be differentiated as an independent segment.

In particular in case of “critical”, sophisticated and/or sensitive technical parts, it may be required to protect, and avoid damage to, structural layers lying, for example, deeper than 1 mm under the external surface, when applying the surface coating. This requirement can be taken into account with a relatively high energy density, high frequencies and a short exposure time of the inductive heating. Advantageously, the operating parameters, i.e. in particular energy density, frequency and/or exposure time, are specifically selected in view of and in consideration of the respective material properties of the carrier body. For example, the most usual carrier materials steel, copper and aluminium have the following thermal conductivities (in W/(mK)): steel 50, copper 300, aluminium 240. Therefore, the operating parameters for the process guidance are advantageously selected so as to be suitably matched to the carrier material, in order to obtain a uniform and reproducible melting-on behavior of the enameling mass.

An MF overall heating and a burning-in on the entire surface is also imaginable in the first step, followed by a second step with another substrate and an HF induction for design purposes.

From the current point of view, the process course and the enameling quality are particularly influenced by:

-   -   material of the carrier and the latter's nature     -   quality of the enameling material (substrate)     -   scope and quality of the pre-treatment steps     -   structure/suitability of the overall equipment     -   selection of the suitable convertor (power, frequency)     -   design of the inductor     -   method parameters of induction:         -   coupling distance         -   power (can also be graduated!)         -   exposure time/heating time         -   in the scanning process:             -   course of movements of the inductor (speed, direction,                 oscillating curve)

In a particularly preferred manner, the course of movements of the inductor relative to the surface to be coated is suitably chosen and adjusted, in view of the other process parameters, such as, for example, treatment temperature, transferred energy or power densities, and the like. In view of material requirements, it can be provided, for example, to avoid excessive temperature differentials in the component to be coated between the currently inductively heated regions, on the one hand, and the currently not heated regions, on the other hand, by passing the inductor fast enough over the respective treated spatial areas. In this way, it is possible to avoid thermal deformations of the piece of material, or at least to keep them small. Linear and/or rotary movements or movement profiles composed thereof are, for example, imaginable movement patterns of the respective inductor over the surface to be treated.

Generally, it is possible to run an enameling program—adapted to the object and the specific requirements—in a variable manner within wide limits and to associate each object point with specific method parameters. This results in a large number of possibilities of design. The following features are in particular considered as particularly advantageous:

-   -   The inductor design can be complex and with wide variations, in         an application-oriented manner, for example round, flat for         scanning, single and multi-turn, depending on object size and         application, meander-shaped area inductor (larger “working         surface”); also of a semicircular, oval or rectangular design,         adapted to the object.     -   As far as process course and enameling quality are concerned,         generally all metals are favorable, particularly preferably         those with good inductive coupling (iron, steel, alloys);         copper, aluminium, precious metals (art!), gold, silver,         platinum.     -   The quality of the enameling material should be chosen in view         of: composition, chemistry, grain sizes, pre-treatment.     -   A particularly preferred pre-treatment of the carrier material         comprises: cleaning, washing, drying, chemical pre-treatment,         degreasing.

A relevant parameter for the selection of the operating parameters in the process guidance is also the depth of penetration of the electromagnetic field into the material surface—usually depending on frequency and temperature. Therefore, especially in view of deeper structure layers possibly requiring protection, treatment temperature and/or operating frequency of the inductor are particularly preferably chosen such that the depth of penetration is at most approx. 1 mm.

For copper as carrier material, this means, for example, that at a treatment temperature of approx. 20° C., the operating frequency of the inductor is chosen to be approx. 10 kHz or more, whereas at a treatment temperature of approx. 100° C., preferably operating frequencies of more than 20 kHz are provided for this criterion. At a treatment temperature of approx. 20° C., however, preferably operating frequencies of more than approx. 500 Hz are provided for steel as carrier material and operating frequencies of more than als approx. 1 kHz, for aluminium.

The energetic advantages are very well understandable and can in each concrete case of application be recorded without any doubt by measuring technology. This results immediately in environmental friendliness because a strong furnace heating—in whatever way—means a high emission of CO₂. Furthermore, the inductive heating allows a particularly good reproducibility in the production process even in case of locally differing requirements imposed on the component in question.

The comparison of three numeric values clearly illustrates the energetic advantages described here:

Type of heating Power transfer in W/cm² Convection (molecular movement, pulling) 0.5 Radiation (furnace, resistance heating) 8 Inductive heating 30,000 (in the part itself, without transfer medium)

Advantageously, an operating frequency of at least 300 kHz is chosen for the inductor. In this way, it can be made sure even in case of varying materials or ambient conditions, the electromagnetic depth of penetration can be kept sufficiently small to limit the heating to the immediate vicinity of the surface.

In another advantageous embodiment, a defective enamel layer already present on the workpiece is repaired with the help of inductive heating. This can be done exactly in line with the requirements and, therefore, in a resource-sparing manner, due to the locally limited and, therefore, very well focusable heat input.

Advantageously, a power density of at least 10 kW/cm² is inductively transferred onto the workpiece via the inductor. Thus, the periods within which the melting temperature of the enamel powder or slip can be reached are particularly short, so that the heat input onto the surface region can be limited and, taken as a whole, remains relatively low.

It is well imaginable that specifically in consideration of the particular properties and advantages of enameled surfaces, the elements used e.g. for large-scale tunnel linings will lead to an enormous stimulation with decreasing production costs, but also for the production of many receptacles (drinking water, medicine, etc.), the positive time and cost factor will have a sales-promoting effect.

By consistently limiting the temperature input to the actual surface regions of the component to be coated, through consistent utilization of the skin effect, it is, furthermore, possible in a particularly advantageous development, to also apply surface coatings in the manner of a composite material, in the sense of a combination of enamel and metal. In this way, or else through suitably chosen enameling coatings as such, it is possible, in a particularly preferred use, to efficiently preserve corrosion-endangered components or components for use in an aggressive environment, such as, for example and particularly preferably, formed parts for hulls below the water line or the like.

In an alternative, particularly advantageous use, which is considered as independently inventive, the concept of inductive heating of a component is used for powder coating/burning in (preferably for window sills, window casement sections, facade elements, fence elements, or other building materialis), for Teflon coating, or for diamond, ceramic and/or crystal coating of surfaces.

One examplary embodiment of the invention is explained in more detail by means of a drawing in which the FIGURE shows an enameling installation for coating a workpiece.

The enameling installation 1 according to the FIGURE is provided for applying a corrosion-protecting enamel coating on a workpiece 2. The enamel coating is produced by applying first of all a suitably selected source material, in particular a so-called enameling paste, on the workpiece 2. Then, the workpiece 2 is heated up in that spatial area in which the application shall be effected, to a temperature above the melting temperature of the enameling material, so that a melting-on of the enameling material sets in. As a working temperature, a minimum temperature chosen as a function of the material of the workpiece 2 is exceeded in the respective treated spatial area, for example, for aluminium as material of the workpiece 2, approx. 500° C. and for steel as material of the workpiece 2, approx. 850° C.

The enameling installation 1 is specifically designed for achieving a high-quality coating result with a homogeneous surface of high quality with a use of particularly little resources, i.e. in particular energy expenditure. For this purpose, the enameling installation 1 is designed for a local or region-wise heating of the workpiece 2 through electromagnetic induction. The enameling installation 1 comprises an induction head or inductor 4, which is connected via an electric line system 6 with an energy-supply unit 8 comprising a converter and a control unit. During operation, the inductor 4 is positioned above and near the surface of the workpiece 2, so that the electromagnetic alternating field radiated from the inductor 4 couples into the surface of the workpiece 2, thus heating the latter.

The inductor 4 can be configured in a multitude of possible variants with regard to its geometric and design parameters. In particular, the lateral extension of the inductor 4, which also determines the size of the surface segment of the workpiece 2 simultaneously heated in each case during operation, can be configured, as a function of the application, so as to be relatively small (few cm² or even less, enabling a locally very differentiated treatment of the workpiece surface during enameling), relatively large (for example 1000 cm² or even more, enabling a relatively large-area and, therefore, quick treatment even of relatively large overall surfaces) or with values lying between these limits.

The enameling installation 1 is designed, for example by means of holding devices, not shown in detail, for the inductor 4 and/or the workpiece 2, for a so-called “scanning mode”, in which the inductor 4 is moved during the enameling process relative to the surface of the workpiece 2 in x and/or y direction (shown in the FIGURE by the arrows 10), thus scanning the surface. In such a mode, the inductor 4 can be moved gradually over the entire surface of the workpiece 2, so that said surface is completely scanned and a complete treatment of the material surface is effected. Alternatively, the inductor 4 can, however, also be activated only on selected parts or segments of the surface of the workpiece 2, which is very advantageous, for example, for repairing damaged surface parts or the like, due to the very requirement-oriented utilization and, therefore, very low overall energy consumption.

The movement of the inductor 4 over the surface of the workpiece 2 can be effected, for example, by means of suitable mobile holding or carrying arms and a suitable automated activation. In an alternative, particularly advantageous embodiment, which is considered as independently inventive, the inductor 4 can, however, also be configured as a portable handheld device, which can be moved over the surface of the workpiece 2 manually.

The enameling installation is configured for a particularly resource-sparing operating mode in the surface treatment of the workpiece 2, wherein both energy consumption and material consumption shall be kept particularly low, achieving, at the same time, a high material quality of the surface. For this purpose, it is provided to consistently make use, during the inductive heating of the workpiece 2, of the so-called skin effect, i.e. the limited depth of penetration of electromagnetic alternating fields into metallic surfaces, for limiting the inductively generated heating as far as possible to the actual surface of the workpiece 2, without producing excessive heating of the deeper layers or spatial areas of the workpiece 2. For this purpose, the operating parameters of the enameling installation 1 are chosen such that—taking into account the material properties of the workpiece 2—the depth of penetration is at most approx. 1 mm.

For copper as carrier material this means, for example, that at a treatment temperature of approx. 20° C., the operating frequency of the inductor is chosen to be approx. 10 kHz or more, whereas, at a treatment temperature of approx. 100° C., preferably operating frequencies of more than approx. 20 kHz are provided for this criterion. At a treatment temperature of approx. 20° C., however, preferably operating frequencies of more than approx. 500 Hz are provided for steel as carrier material, and operating frequencies of more than approx. 1 kHz, for aluminium.

In order to ensure a genuine surface effect even under varying operating conditions, an operating frequency of at least 300 kHz is chosen. In this way, it is achieved that the depth of penetration can be kept sufficiently small under all expectable conditions, so that the heating can be kept limited to the immediate surface area and deeper structural layers are not appreciably exposed to the heating.

The remaining operating parameters are also suitably chosen in view of the provided resource-sparing operating mode. The inductor 4 is in particular operated with a power density of approx. 10 kW/cm² (referred to the radiating surface). Especially in combination with the provided small depth of penetration into the workpiece 2, this means that the spatial area to be heated in the surface region of the workpiece 2 is exposed to a high power density, so that the necessary treatment times, i.e. in particular up to reaching the melting temperature of the enamel paste at the surface, can be kept particularly short.

A particularly advantageous aspect of such a selection of parameters consists in the fact that due to the heating being specifically directed onto the surface, even relatively thin coatings can be generated, whose material properties, such as, for example, elasticity etc., are oriented according to the substrate or carrier body. 

1. A method for producing a completely or partially enameled component, wherein the workpiece (2) carrying an enamel powder is inductively heated, the operating frequency of the inductor (4) being chosen, in view of the material properties of the workpiece (2), such that the electromagnetic depth of penetration into the workpiece (2) is at most 1 mm.
 2. The method of claim 1, wherein an operating frequency of at least 300 kHz is chosen for the inductor (4).
 3. The method of claim 2, wherein a defective enamel layer already present on the workpiece (2) is repaired by means of the inductive heating.
 4. The method of claim 3, wherein a power density of at least 10 kW/cm² is inductively transferred to the workpiece (2) via the inductor (4).
 5. The method of claim 2, wherein a power density of at least 10 kW/cm² is inductively transferred to the workpiece (2) via the inductor (4).
 6. The method of claim 1, wherein a defective enamel layer already present on the workpiece (2) is repaired by means of the inductive heating.
 7. The method of claim 6, wherein a power density of at least 10 kW/cm² is inductively transferred to the workpiece (2) via the inductor (4).
 8. The method of claim 1, wherein a power density of at least 10 kW/cm² is inductively transferred to the workpiece (2) via the inductor (4). 